In a hot strip mill a hot steel strip travels along rollers from a heating furnace to a rolling mill. During the time taken to travel this distance a certain amount of cooling will take place which will result in the exposed surfaces of the steel strip having a lower temperature than the interior of the strip. This cooling effect is most pronounced at the edges of the strip and it is common to find that the edges are 50.degree. to 70.degree. cooler than the centre of the strip. This cooling has an adverse effect on the steel during the rolling process and can cause the rolled steel produced to have cracked or mis-shapen edges.
In most cases it is not possible to site the furnace adjacent to the rolling mill as a gap has to be provided between the furnace and the rolling mill at least equal to the maximum length of the steel strips produced. This gap will allow the rolling mill to be stopped quickly in the event of a problem occurrring but still allow steel strips to be removed from the furnace.
Known methods of preventing the loss of temperature to the edges of the strip include heating the strip to a higher temperature than normal by increasing the residence time of the strip within the furnace or by operating the furnace at a higher temperature. As the strip will leave the furnace at a high temperature any cooling that takes place will be insufficient to reduce the temperature of the edges to a point at which rolling cannot be performed effectively. However if a larger residence time within the furnace is allowed the process is slowed down correspondingly and if the furnace operating temperature is raised increased costs are incurred as a result of a lower operating efficiency.
A further method is to install secondary heaters, effective on the edges of the strip, immediately prior to the rolling mill. These secondary heaters will raise the temperature of the edges of the strip to an acceptable level for the rolling process to take place. In practice it is known to use such heaters in pairs positioned above and below respective edges of the steel plate. Such secondary heaters are generally inefficient compared to the furnace and the size of the secondary heater is limited by the space between the rollers used to convey the steel strip from the furnace to the rolling mill.